Causes Of N. Atlantic Ocean’s Rapid Warming In The Mid-90s
Most of us are probably familiar with the pattern of Arctic sea ice decline between 1979 and 2007, followed by a period of relative stability.
Most of the decline took place after the mid-1990s.
The decline is nearly always explained away as the result of global warming, but a couple of old studies show this not to be the case.
In 2011, Robson & Sutton found that the subpolar gyre underwent remarkable and rapid warming in the mid-1990s and that this was linked to changes in the North Atlantic Oscillation:
A gyre is simply a system of rotating ocean currents:
This is the key chart in their paper:
Graphs a) and b) compare sea surface temperatures, and show just how radically seas warmed between the two decades in the N Atlantic.
Perhaps even more remarkable are the temperatures for the oceans down to 500m, shown in c) and d).
There is no physical basis for such enormous changes to have been caused by greenhouse gases because the heat capacity of the oceans is far too massive. The explanation must lie elsewhere.
The Robson study fits in with an earlier one from NASA in 2007, which linked climatic changes in the Arctic to the Arctic Oscillation:
PASADENA, Calif. – A team of NASA and university scientists has detected an ongoing reversal in Arctic Ocean circulation triggered by atmospheric circulation changes that vary on decade-long time scales. The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming.
The team, led by James Morison of the University of Washington’s Polar Science Center Applied Physics Laboratory, Seattle, used data from an Earth-observing satellite and from deep-sea pressure gauges to monitor Arctic Ocean circulation from 2002 to 2006.
They measured changes in the weight of columns of Arctic Ocean water, from the surface to the ocean bottom. That weight is influenced by factors such as the height of the ocean’s surface, and its salinity. A saltier ocean is heavier and circulates differently than one with less salt.
The very precise deep-sea gauges were developed with help from the National Oceanic and Atmospheric Administration; the satellite is NASA’s Gravity Recovery and Climate Experiment (Grace). The team of scientists found a 10-millibar decrease in water pressure at the bottom of the ocean at the North Pole between 2002 and 2006, equal to removing the weight of 10 centimeters (four inches) of water from the ocean. The distribution and size of the decrease suggest that Arctic Ocean circulation changed from the counterclockwise pattern it exhibited in the 1990s to the clockwise pattern that was dominant prior to 1990.
Reporting in Geophysical Research Letters, the authors attribute the reversal to a weakened Arctic Oscillation, a major atmospheric circulation pattern in the northern hemisphere. The weakening reduced the salinity of the upper ocean near the North Pole, decreasing its weight and changing its circulation.
“Our study confirms many changes seen in upper Arctic Ocean circulation in the 1990s were mostly decadal in nature, rather than trends caused by global warming,” said Morison.
“While some 1990s climate trends, such as declines in Arctic sea ice extent, have continued, these results suggest at least for the ‘wet’ part of the Arctic — the Arctic Ocean — circulation reverted to conditions like those prevalent before the 1990s,” he added.
The Arctic Oscillation was fairly stable until about 1970, but then varied on more or less decadal time scales, with signs of an underlying upward trend, until the late 1990s, when it again stabilized. During its strong counterclockwise phase in the 1990s, the Arctic environment changed markedly, with the upper Arctic Ocean undergoing major changes that persisted into this century.
Many scientists viewed the changes as evidence of an ongoing climate shift, raising concerns about the effects of global warming on the Arctic.
Morison said data gathered by Grace and the bottom pressure gauges since publication of the paper earlier this year highlight how short-lived the ocean circulation changes can be. The newer data indicate the bottom pressure has increased back toward its 2002 level. “The winter of 2006-2007 was another high Arctic Oscillation year and summer sea ice extent reached a new minimum,” he said. “It is too early to say, but it looks as though the Arctic Ocean is ready to start swinging back to the counterclockwise circulation pattern of the 1990s again.”
The Arctic Oscillation is closely linked to the NAO, and is said to be “negative” when high pressure dominates the pole:
According to NSIDC:
The Arctic Oscillation primarily affects sea ice through winds that cause changes in where the sea ice drifts.” When the Arctic Oscillation is in its negative mode, he said, the winds and ice tend to flow in a clockwise direction, generally keeping more of the older, thicker ice in the middle of the Arctic. In the positive phase, that old ice tends to get pushed out of the Arctic along the Greenland coast. Meier said, “This means that the sea ice tends to be younger and thinner and more prone to melt after a winter with a strong positive Arctic Oscillation
The unusually positive AO is evident in the late 1990s, since when it has been more stable.
Note also positive AO during the 1920s, when the Arctic underwent similar warming.
The exact mechanisms are complex and still not well understood by scientists. But both of these studies point to natural, decadal atmospheric changes as being responsible for Arctic sea ice trends since the 1990s, which include the self-evident stabilization of sea ice extent since 2007.
Arctic image by Free-Photos from Pixabay
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jerry krause
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Hi Paul,
You reported: “The Arctic Oscillation primarily affects sea ice through winds that cause changes in where the sea ice drifts.” When the Arctic Oscillation is in its negative mode, he said, the winds and ice tend to flow in a clockwise direction, generally keeping more of the older, thicker ice in the middle of the Arctic. In the positive phase, that old ice tends to get pushed out of the Arctic along the Greenland coast. Meier said, “This means that the sea ice tends to be younger and thinner and more prone to melt after a winter with a strong positive Arctic Oscillation.”
What I have yet to read anywhere, although I must admit I have not an exhaustive literature search relative to the “The Arctic Oscillation primarily affects sea ice through winds that cause changes in where the sea ice drifts.”, the influence of the centrifugal effect of the rotating Earth upon the drift of the ice sheets.
At (https://follow.mosaic-expedition.org/) is a map of the Arctic Circle region in which the ice sheet is pushed up against the northern coast of Greenland and the Canadian Arctic Archipelago at the beginning of September of 2019. Obviously, the phase of the Arctic Oscillation was going to be a positive phase. And the MOSAiC epedition was planned to be a repeat of Nansen’s 1893 to ???? expedition for which he built as special ship (the Fram) to freeze in the ice sheet and drift with the ice sheet to the North Pole.
But in Nansen’s case the phase of the Arctic Oscillation was negative as the Fram’s drift became frozen in at about 78oN while the icebreaker Polarstern of the MOSAiC project became frozen in at about 86oN near the 140oE which both expeditions maybe hoped to drift along to the North Pole. .Now a fact is the centrifugal effect is about 9 times stronger at 78oN than it is at 86oN.
Now it seemed simple to reason the prevailing winds during the winter should drain of the huge north slope of Russia along the river valleys which drained melt water from the higher elevations of the Asian Continent. And the mouth of the Lena River, which had the largest watershed of the rivers which drained into the Arctic Ocean, was near the 140oE longitude.
So the situation for Nansen was that as the prevailing wind pushed the ice sheet toward the pole and centrifugal effect ‘pushed’ back on the ice sheet. Again, I must repeat I have not yet read about this simple, obvious analysis. Hence, I cannot believe others have not seen the same things that I just reviewed.
But to continue this analysis a bit further, first I see that during the late spring and summer as the ice sheet melts, land surfaces at lower latitudes will warm more than the surfaces of the ocean, and the prevailing winds would reasoning be from the pole toward the continents if the September ice sheet. In the case the winds and centrifugal effect are ‘pushing’ together. But if Nansen could not get closer to the pole than 78oN it becomes obvious, if one studies a map, that the ice sheet along the 140oE longitude extended to 40oW longitude, is not centered on the pole and the ice sheet will be ‘pulled’ away from Greenland by the unbalanced centrifugal effect.
Now Meier said, “This means that the sea ice tends to be younger and thinner and more prone to melt after a winter with a strong positive Arctic Oscillation.” Now, it one looks at the extent of ice at the beginning of the MOSAIC project is obvious that most the new ice is going to form along the 140oE latitude. And what Meyer only indirectly states is that the ice against Greenland has logically been piled up to greater depths that no icebreaker can penetrate. Hence, the centrifugal effect of the greater extent of the new ice toward Russia is balanced by the greater mass (depth) of the ice along Greenland and along the Canadian Arctic Archipelago at the beginning of September 2019. And this ice toward the ‘west extends further and further away from the pole and the centrifugal effect of this ice further stabilizes the position of the small ice sheet which remained in September of 2019.
What will be very interesting to see is what remains at the beginning of September 2020. And do not consider the analysis of this Arctic Ocean and surrounding continents system complete. For there is more to imagine and then reason.
Have a good day, Jerry
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